1. Field of the Invention
The present invention relates to drier apparatus for drying sheets of photosensitive material, and more particularly to a drier for drying a photosensitive material processed with processing solutions while the photosensitive material is being transported.
2. Description of the Related Art
Hitherto, drier apparatus has been proposed in which a photosensitive material is wound around a heat roller heated by a heater and the photosensitive material is dried by heating. The adjustment of the surface temperature of this heat roller has been carried out by the so-called on-off control. This on-off control is effected by detecting the surface temperature of the heat roller by means of a temperature sensor disposed in the vicinity of the surface of the heat roller, and by turning on and off the heater in correspondence with the temperature difference with respect to a set temperature such that the surface temperature of the heat roller becomes the set temperature. With this method, however, there have been drawbacks in that the loss of heat through contact with the photosensitive material causes a decline in the surface temperature of the heat roller, and that the surface temperature, after declining, can rise excessively high, so that the surface temperature of the heat roller fluctuates substantially and the deviation from the targeted temperature becomes large, thereby making it difficult to dry a plurality of photosensitive materials on a stable basis.
With the conventional on-off control, there have been cases where the finished quality of the photosensitive material after being dried varies depending on the processing conditions of the photosensitive material, e.g., the timing of insertion of each photosensitive material in a case where a plurality of photosensitive materials are processed. That is, the degree of fluctuation of the surface temperature differs in cases where the plurality of photosensitive materials are inserted continuously without pauses and in cases where they are inserted with arbitrary intervals of time. Since the deviation from the targeted temperature thus differs, the finished quality after drying is not constant. Furthermore, there has been a problem in that, in cases where a plurality of photosensitive materials of different types, such as a plurality of photosensitive materials having different drying properties, are inserted in mixed form, the finished quality after drying sometimes differs depending on the types of photosensitive material.
Furthermore, with the progress made in the field of electronics, speedy processing has been required in the field of silver halide photography as well. There has been a growing demand for speedy processing of such photosensitive materials as graphic arts photosensitive materials, photosensitive materials for scanners, and X-ray photosensitive materials, in particular. The speedy processing referred to herein means processing in which the time duration from the time a leading end of the photosensitive material is inserted into a photosensitive material processor, such as an automatic processor, until the time the leading end of the photosensitive material is discharged from a drying station after passing through a processing station consisting of a developing tank, a fixing tank, a washing tank, etc., and the drying station is, for instance, 20 to 60 seconds. If the transporting speed of the photosensitive material is merely made faster to reduce the processing time of the processing and drying stations, various problems such as faulty fixation and faulty drying occur.
With respect to the faulty fixation, a technique is known in which the concentration of thiosulfate in the fixing solution is increased so as to increase the fixing speed. In addition, a technique in which a hardening agent such as water soluble aluminum salts is contained in the fixing solution is also known and widely practiced. However, if the photosensitive material is processed with the fixing solution containing the hardening agent such as water soluble aluminum salts, the fixing speed is delayed due to the hardening action. Therefore, if an attempt is made not to virtually contain the hardening agent in the fixing solution so as to increase the fixing speed, the swelling rate of the emulsion coated on the photosensitive material becomes disadvantageously large, resulting in deteriorated drying properties. Thus, reducing the quantity of the water soluble aluminum salts, i.e., the hardening agent, in the fixing solution improves the fixing speed, but acts disadvantageously in terms of the reduction of the drying time which is important in speedy processing. Accordingly, there have been practically no attempts to process the photosensitive materials by using the fixing solutions which virtually do not contain the hardening agents.
Here, to realize speedy processing, it is effective to dry the photosensitive material processed with a fixing agent which virtually does not contain the hardening agent, by means of the heat roller. However, a defect similar to the above-described one occurs if the temperature adjustment of the heat roller is effected by the so-called on-off control in which control is effected by detecting the surface temperature of the heat roller and by turning on and off the heater in correspondence with the temperature difference with respect to a set temperature such that the surface temperature of the heat roller becomes the set temperature.
The drier of the type in which drying is effected by winding the photosensitive material around a heat roller has a plurality of heat rollers, and obverse and reverse surfaces of the photosensitive material are respectively wound around different heat rollers so as to dry both surfaces. Furthermore, a temperature sensor is provided for each heat roller, and control of the surface temperature of each heat roller is effected independently. More specifically, the temperatures detected by the temperature sensors are retrieved at a fixed timing, a time duration for supplying power is determined for each heater so that the surface temperature of each heat roller can be maintained at the set temperature, and relays and the like provided between the heaters and a power source are controlled so that power is supplied to the heaters for the aforementioned time durations of power supply.
However, with the above-described temperature control, there are cases where power is supplied simultaneously to the heaters depending on the lengths of the durations of power supply for the respective heaters. For example, in FIG. 16A, power supply to a heater S1 is started with a fixed period, and power supply to a heater S2 is started independently of the period of power supply to the heater S1. In cases where, for instance, the difference in the time when power supply to the respective heaters is started is small, if the time duration of power supply to the heater S1 is made longer than the difference t.sub.0 in the time when power supply to the respective heaters is started so as to increase the surface temperature of the heat roller in a short time, there occurs a period when the period of power supply to the heater S1 and the period of power supply to the heater S2 overlap.
In cases where power is supplied to a plurality of heaters, the plurality of heaters are generally connected to the power source in parallel. Therefore, during the period when the power-supplying periods thus overlap, the sum of values of currents flowing across the heaters increases to a high level, as shown in FIG. 16B. In power-supplying means which consists of a power-supplying line, a power source, and the like for supplying electric power to a current load such as a heater, the allowable current of the power-supplying line, the capacity of the power source, and so on are generally set by taking into consideration the peak value of current flowing across the load. In conventional driers, a plurality of heaters are electrically connected to the power-supplying means, and in some driers a plurality of heaters are arranged on the inner periphery of each heat roller. Since the power-supplying means may possibly supply electric power simultaneously to the plurality of heaters, the peak value of current flowing across the plurality of heaters becomes very high, as described above.
Accordingly, it is necessary to increase the allowable current of the power-supplying line of the power-supplying means and increase the capacity of the power source. Also, it is necessary prepare power-supplying systems in a number equal to or greater than the number of the heat rollers in the case where a plurality of heaters are arranged within each heat roller. Hence, the power-supplying means has been high in cost.